Breastfeeding perfectly but won’t bottle feed

You might well be right, but heaven forbid you should say it!

Perhaps suggest Mum expresses so she can then use a bottle (and explain in world weary tones to her H that its the only way to get baby ready for child care scenario) and see what happens? Its devilishly difficult offering advise with a baby especially when it tantamount criticises one of the parents.
In the great scheme of things its probably best that you keep quiet about it but just be prepared to try a different range of bottles. Many years ago I had trouble getting a DD off the boob - after 6 months my body wanted to be alone. She did not take to standard bottles and teats but I found a weird one that had a very wide spread teat that felt more realistic to her, not sure if they are still available. But to be honest to be in for the long haul i.e. being a good dependable Grandma...you need to just be silent but supportive of all their efforts (even if you want to scream, PULL YOUR WEIGHT YOU PLONKER)

We’ve new babies in the family and I know my GD is determined to get the baby breastfeeding but will introduce the bottle too so that her partner can feed and she will be able to leave him with family in a few months without issues with feeding.
I don’t think you are wrong to encourage your DD to get him used to the bottle for everyone’s sake.